The present invention relates generally to radiocommunication methods and systems and, more particularly, to the processing of code division multiple access (CDMA) signals.
The explosive growth in radio telecommunications requires continual improvement in the capacity, flexibility and quality of such systems. Evolution in this arena has seen the movement from analog to digital technology and from frequency division multiple access (FDMA) to time division multiple access (TDMA). Code division multiple access (CDMA) provides some special features as compared with current TDMA standards and may well be a good choice for future third generation systems. While changing among different technologies has increased capacity, improving the efficiency of system components is also necessary to provide the degree of additional capacity required by consumer demand. Thus, for example, continual efficiency improvements are being achieved in the transmitters and receivers of radiocommunication systems.
In receivers, reliable detection of data is dependent upon the ability of the receiver to resolve ambiguities in received signals. A receiver with superior detection performance relaxes the constraints of other parts of the communication system which translates into increased capacity, smaller size, reduced cost or some other advantage. For example, base stations with improved reception capabilities will allow mobile stations to use lower transmit power, resulting in immediate savings in battery power. Alternately, the improved signal detection can also be used to, for example, improve link quality.
Coherent detection, whereby the change of phase induced in a received radio signal by the radio channel is undone by correlation to a reference signal, is preferable for receivers to non-coherent detection which derives an estimate of the sought signal from both quadratures and thereby faces a loss of up to 3 dB relative to coherent detection. Thus, coherent rather than non-coherent detection is typically the preferred choice. Techniques for both coherent and non-coherent detection are well known, but their usage depends on the availability of reference signals. If a reference signal exists, coherent detection is the typical choice, otherwise non-coherent detection must be used. The reference signal can be a dedicated pilot signal or simply pilot symbols known to the receiver which are interspersed with the data signal.
Systems based on Time Division Multiple Access (TDMA) often incorporate pilot or reference symbols interspersed with data symbols. When information in one link is decoded, information in each time slot is considered individually, including the reference information. This reference information can either be lumped together in one portion of the time slot or spread throughout. In either case, the reference information is time multiplexed with user data. Various schemes are used to estimate the radio channel at points within the slot where no reference samples are available.
In direct sequence Code Division Multiple Access (CDMA) systems, transmission is continuous, i.e., users are distinguished through different codes rather than different combinations of carrier frequency and time slot as in TDMA systems. In the recently announced U.S. standards for CDMA systems, IS-95, coherent detection is assumed in the downlink of the system, i.e., from the fixed base station down to the mobile station. Since a base station can easily pool resources, all mobile stations in a cell share one pilot signal which provides a phase reference for coherent detection. Often, this pilot signal is transmitted with more power than the signals unique to each base station-mobile station link. Thus, a reliable reference of the radio channel is established.
Coherent detection is not, however, suggested in IS-95 for the uplink since coherent detection would not be supported in the uplink by any reference signals. In the uplink, signals from mobile stations traverse unique radio channels and thus a common pilot resource is not available. Moreover, the uplink modulation method is M-ary orthogonal signaling, which can easily be detected non-coherently by a square-law detector. Although coherent detection of an M-ary orthogonal signal per se can be achieved, such detection comes only with a considerable increase in complexity.
Coherent detection can also be obtained in DS-CDMA with pilot symbol aided detection techniques as described in P. Hoeher, "Tradeoff between Channel Coding and Spreading for DS-CDMA," RACE Mobile Telecommunications Workshop, Metz, June 1993. This approach is also problematic since, due to the fast variations of the channel, it would be necessary to distribute the pilot symbols over an entire frame. In a CDMA system with variable bitrates and spreading factors it would become complicated to introduce pilot symbols interleaved with the data sequence.
Absent some way in which coherent detection can be provided in the uplink, it will be necessary to use non-coherent detection and accept the corresponding reduction in system performance. Accordingly, it would be desirable to provide a system and method for coherent detection in, for example, the uplink of CDMA systems without introducing additional pilot symbols or signals.